High-carbon steel continuous casting billet center quality control method

Abstract

The invention relates to a high-carbon steel continuous casting billet center quality control method, and belongs to the technical field of high-carbon steel continuous casting billet preparation methods. In order to solve the problems that an existing high-carbon steel continuous casting billet is prone to being low in center density, obvious in porosity and shrinkage cavity defect and serious in segregation, the center quality control method for the high-carbon steel continuous casting billet is provided, a converter / electric furnace-refining-vacuum-continuous casting technology is adopted, and the C content, the P content and the S content of finished steel are controlled; according to the tundish molten steel superheat degree, the pulling speed is controlled in a linkage mode, the crystallizer cooling water amount is 2450 L / min, the crystallizer electromagnetic stirring parameter is 150 A / 2 Hz, and the tail end electromagnetic stirring parameter is 400 A / 6 Hz. According to the method, smelting-continuous casting full-process closed-loop control is adopted, parameters of all working procedures are cooperatively regulated and controlled, impurities are controlled from the source, the superheat degree and the pulling speed are accurately controlled, center segregation, looseness, shrinkage cavities and microcracks of the high-carbon steel continuous casting billet are effectively restrained, and the quality stability and the machining qualification rate of the casting billet are remarkably improved.

Description

A method for center quality control of high carbon steel continuous casting billets Technical Field

[0001] This invention belongs to the technical field of high-carbon steel continuous casting billet preparation methods, and particularly relates to a method for center quality control of high-carbon steel continuous casting billets. Background Technology

[0002] High-carbon steel is an indispensable key engineering material in the industrial field. With its core mechanical properties such as high strength, high hardness, and excellent wear resistance, it is widely used in machinery manufacturing, precision tooling, industrial mold production, and the manufacture of heavy-duty structural components. Thanks to its outstanding performance advantages, high-carbon steel is suitable for harsh working conditions involving high loads and intense wear, making it an ideal material for manufacturing various heavy-duty wear-resistant parts, directly determining the service life and reliability of end products.

[0003] Continuously cast billets are a crucial intermediate product in the high-carbon steel production process, and the quality of their central region plays a decisive role in the final performance of the finished product. The density, chemical composition uniformity, and degree of internal defects such as central porosity, shrinkage cavities, and segregation of the billet center directly affect the mechanical properties, service life, and stability of the finished product after subsequent rolling and forging. Substandard central quality can lead to problems such as cracking during subsequent processing and unqualified finished product performance, significantly reducing the product qualification rate.

[0004] Compared to low-carbon steel and ordinary alloy steel, high-carbon steel continuously cast billets, due to their higher carbon content, exhibit unique solidification metallurgical behavior that significantly increases the difficulty of central quality control. Various defects accumulate and multiply, creating multiple intractable quality problems. High-carbon steel experiences greater solidification phase transformation shrinkage and a wider solid-liquid two-phase region. Dendritic crystals in the pasty region easily block the central feeding channels. Insufficient timely feeding of molten steel directly leads to central porosity and shrinkage cavities. Furthermore, these inherent defects cannot be completely eliminated during subsequent hot working, leaving residues that pose performance risks. Simultaneously, the high carbon content significantly exacerbates solute segregation, with excessive enrichment of carbon and impurity elements at the solidification front, resulting in severe central component segregation. This leads to uneven distribution of the billet's internal structure and properties, making it difficult to meet the requirements of high-end materials. In addition, the superposition of solidification shrinkage stress, thermal stress, and segregation-induced local stress easily induces central microcracks, which in turn interact with porosity and segregation areas, accelerating defect propagation and deterioration. Multiple defects trigger each other and create a vicious cycle, further amplifying the difficulty of quality control for high-carbon steel continuous casting billets, becoming a prominent production problem in the industry. Summary of the Invention

[0005] To address the problems of low center density, significant porosity and shrinkage defects, and severe segregation in existing high-carbon steel continuous casting billets, this invention provides a method for center quality control of high-carbon steel continuous casting billets.

[0006] The technical solution of the present invention:

[0007] A method for controlling the center quality of high-carbon steel continuously cast billets adopts a converter / electric furnace-refining-vacuum-continuous casting process. The composition of the finished steel is controlled as follows: C not less than 0.6%, P ≤ 0.010%, S ≤ 0.05%. The casting speed is controlled in conjunction with the superheat of the molten steel in the tundish: 0.65 m / min when the superheat is <15℃, 0.60 m / min when the superheat is 15~45℃, and 0.55 m / min when the superheat is >45℃. The cooling water flow rate of the crystallizer is 2450 L / min, the electromagnetic stirring parameter of the crystallizer is 150 A / 2 Hz, the electromagnetic stirring parameter of the end is 400 A / 6 Hz, and the alternating stirring cycle is 10 s running-3 s pause-10 s running-3 s pause. The static light reduction parameters are 3 / 5 / 5 / 5 / 5 mm, and the total reduction is 23 mm. After the start of continuous casting, the flow is manually controlled for no less than 60 s before automatic control of the liquid level is introduced. During the ladle changing process, the tundish is kept full for casting.

[0008] Furthermore, the initial composition of the molten iron meets the following requirements: C not less than 4.0%, Si: 0.30~0.60%, S≤0.040%, Ti≤0.060%, P≤0.080%, and the temperature of the molten iron entering the furnace is ≥1300℃.

[0009] Furthermore, the superheat of the molten steel is controlled within the range of 25~35℃.

[0010] Furthermore, the refining time is no less than 60 minutes, and the total time from the end of refining to the hoisting of molten steel to the continuous casting pouring position is consistent with the single-heat pouring cycle of the continuous casting machine, to ensure that the molten steel level in the tundish does not fluctuate significantly and the superheat is stable, so as to achieve uninterrupted continuous pouring of multiple heats.

[0011] Furthermore, the length of the tundish nozzle is 640±5mm, and the nozzle spacing is 1300±5mm.

[0012] Furthermore, the intermediate ladle adopts a gradient baking process: the low-heat baking time is not less than 2.5 hours, the low-heat baking temperature is ≤300℃, the flame covers 1 / 3 of the ladle wall for the first hour of low-heat baking, and the flame covers 2 / 3 of the ladle wall for 1~2.5 hours of low-heat baking; the medium-heat baking is carried out until the flame reaches the bottom of the ladle and produces back flame and the working layer at the bottom of the ladle turns red, and then the high-heat baking with air is turned on to bake to the specified heat storage temperature; the total baking time of low-heat, medium-heat and high-heat baking is ≤8 hours; the stopper rod is baked separately for ≤4 hours, and the immersion gate is baked separately for ≤3 hours.

[0013] Furthermore, Y-type filters are installed in the continuous casting spray pipes, and the spray water quality is controlled to be pH 10~12, turbidity ≤30NTU, and calcium hardness ≤400mg / L.

[0014] Furthermore, before continuous casting begins, the spray pipes are drained of sewage and water to complete the spray test operation and ensure that the spraying state of each nozzle is uniform.

[0015] Furthermore, a special mold protective slag for large square billets and high carbon steel, model STH-3, is added to the crystallizer; the main components of the mold protective slag include CaO 22~25wt%, SiO2 30~35wt%, Al2O3 3~5wt%, MgO 1~3wt%, and F. - 5~7wt% and fixed carbon 11~15wt%, melting point 1075-1100℃, add frequently, in small amounts, and evenly, so that the slag surface does not leak red steel.

[0016] Furthermore, a special covering agent for high-carbon steel continuous casting tundish is used. The main components of the tundish special covering agent include CaO 3~5wt%, SiO2 44~50wt%, Al2O3 3~4.5wt%, fixed carbon 20~22wt%, and MgO 11~13wt%, with a melting point of 1350℃. After the liquid level in the tundish reaches 200mm, 20kg of the covering agent is added to each stopper rod. After the tundish is full, 20kg is added to each side of the long nozzle. After each furnace is started casting, 10kg is added to each side.

[0017] The beneficial effects of this invention are:

[0018] This invention employs a closed-loop control process encompassing smelting, refining, tundish pretreatment, and continuous casting, with parameters at each stage working in synergy to control center segregation in high-carbon steel continuously cast billets from the source. By ensuring the finished steel has a phosphorus content ≤0.010% and a sulfur content ≤0.05%, combined with ladle refining homogenization treatment, the selective crystallization and enrichment of carbon and harmful elements at the solid-liquid interface are suppressed. This prevents the associated association between segregation, porosity, and cracking, and stably controls the center segregation index of the billet to ≤1.05. This solves the problems of uneven composition and dispersed microstructure and properties in the center of high-carbon steel continuously cast billets, meeting the requirements for high-end applications.

[0019] Based on source impurity control, the precise control of tundish superheat in conjunction with continuous casting speed, electromagnetic stirring, and static reduction solves the technical problem of insufficient solidification feeding in high-carbon steel. This process can narrow the wide solid-liquid two-phase region of high-carbon steel, open up the central feeding channels blocked by dendrites, reduce central porosity and shrinkage defects caused by large solidification phase transformation shrinkage, and avoid residual defects affecting subsequent processing. Ultimately, it achieves central porosity ≤ Grade 1 and shrinkage cavity ≤ Grade 0.5 in the billet, effectively improving the central density of the billet.

[0020] By coordinating and controlling multiple parameters, this invention counteracts shrinkage stress, thermal stress, and segregation stress during the solidification process of the billet, inhibits the initiation and propagation of central microcracks, blocks the path of coordinated deterioration of various defects, and overcomes the quality control challenges posed by the special solidification metallurgical behavior of high-carbon steel. Compared with conventional continuous casting processes, this invention improves the quality stability and subsequent processing qualification rate of high-carbon steel continuous casting billets, enabling large-scale and stable production of high-carbon steel continuous casting billets, and has significant technological advancement and industrial application value. Attached Figure Description

[0021] Figure 1 is a physical image of the cross-section of the high-carbon steel continuous casting billet prepared in Example 2;

[0022] Figure 2 is a physical image of the cross-section of the high-carbon steel continuous casting billet prepared in Comparative Example 1. Detailed Implementation

[0023] The technical solution of the present invention will be further described below with reference to embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention. In the following embodiments, the process equipment or apparatus not specifically specified are all conventional equipment or apparatus in the art. Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commercially available; unless otherwise specified, the technical means used in the embodiments of the present invention are all conventional means well known to those skilled in the art.

[0024] Example 1

[0025] This embodiment provides a method for controlling the center quality of high-carbon steel continuous casting billets. It adopts a standard process route of converter / electric furnace smelting-refining-vacuum, with intermediate ladle pretreatment and a five-machine, five-strand 250mm×280mm continuous casting machine to complete billet production. A closed-loop quality control process is established throughout the entire process. The specific control methods are as follows:

[0026] Prepare the raw materials according to fixed specifications, adding 82±2 tons of molten iron and 25±2 tons of scrap steel per heat. Strictly control the initial indicators of raw materials. The composition of molten iron must meet the following requirements: C≥4.0%, Si: 0.30~0.60%, S≤0.040%, Ti≤0.060%, P≤0.080%. The temperature of molten iron entering the furnace should not be lower than 1300℃. By stabilizing the furnace charge ratio and initial composition, the fluctuation of raw materials is avoided to prevent the subsequent solidification process from exacerbating the separation and crystallization segregation, thus ensuring the quality of molten steel from the source. Subsequently, a converter is used for smelting, and the steel output is controlled to be stable at 100±2t. After smelting, the steel enters the refining and vacuum treatment processes in sequence.

[0027] As a core step in steel homogenization, the refining process strictly controls the refining time to no less than 60 minutes, ensuring that the refining rhythm is highly matched and synchronized with the subsequent continuous casting speed. Simultaneously, precise control of the target composition of the finished steel is crucial, maintaining carbon content at ≥0.6%, phosphorus at ≤0.010%, and sulfur at ≤0.05%. By reducing the content of harmful elements such as phosphorus and sulfur, their enrichment at the solidification front is minimized, mitigating the problem of center segregation from a compositional design perspective. Extending the refining time promotes the full flotation and removal of inclusions in the molten steel, improving its purity and preventing inclusion aggregation that could induce center cracks. It also stabilizes the steel composition and pouring temperature, preventing solidification process disruptions caused by significant temperature fluctuations.

[0028] During the steel refining process, the preparation of the tundish and the gradient baking operation are completed simultaneously. First, the parameters of the tundish nozzle are accurately measured to ensure that the length of the nozzle is controlled at 640mm±5mm and the nozzle flow spacing is 1300mm±5mm. After the parameters are qualified, the tundish is hoisted to the baking position for graded baking. The baking rhythm and temperature are strictly controlled: the baking time of the small fire is not less than 2.5 hours and the baking temperature does not exceed 300℃. After the small fire starts, the flame should cover 1 / 3 of the ladle wall in 1 hour. After the small fire lasts for 1 to 2.5 hours, the flame should extend to 2 / 3 of the ladle wall. After the medium fire is baked until the flame reaches the bottom of the ladle and backfires and the working layer at the bottom of the ladle turns red, the large fire baking can be turned on. The large fire is baked with air until the tundish reaches the specified heat storage temperature. The baking time for each stage of low, medium, and high heat can be appropriately extended, but the total baking time shall not exceed 8 hours. The baking time for the stopper rod alone shall not exceed 4 hours, and the baking time for the submerged nozzle shall not exceed 3 hours. Gradual baking stabilizes the overall temperature of the tundish, effectively controls the temperature drop of molten steel during casting, and avoids the sudden drop in overheating leading to unbalanced solidification shrinkage and insufficient feeding, which can cause central porosity and shrinkage defects.

[0029] After the intermediate tundish is prepared, pretreatment of the continuous casting auxiliary system is carried out in advance. Y-type filters are installed between the nozzles of the continuous casting spray pipes, and a regular replacement system is established. Before production starts, the spray pipes are thoroughly drained and treated to ensure that the spray water quality meets the standards. The specific indicators are pH value 10~12, turbidity ≤30NTU, and calcium hardness ≤400mg / L, so as to prevent nozzle clogging problems from the source. After the spray system is debugged, a test water operation is carried out to check the spray status of each nozzle one by one to ensure that the spray is uniform throughout the continuous casting process. This avoids excessive temperature difference and stress concentration inside and outside the billet caused by local overcooling or overheating. In view of the poor high-temperature plasticity of high-carbon steel, the initiation and propagation of central cracks are prevented from the cooling process.

[0030] After completing all preliminary preparations, the standardized start-up and steady-state pouring process for continuous casting begins: After precise alignment of the tundish nozzle, the stopper rod adjustment status is confirmed. Then, the ladle arm is rotated to the pouring position, the continuous casting main control activates the sliding nozzle, and after confirming the signal is normal, the on-site operators are notified to switch the on-site control box to pouring mode, officially starting the pouring process. During the initial pouring stage, the billet pulling speed is increased to the process setting value. Once the molten steel level in the crystallizer stabilizes and the submerged entry nozzle shows a normal reddish tinge, manual flow control is maintained for at least 60 seconds. After confirming stable operating conditions, automatic liquid level control is then implemented to ensure a high success rate for the first pouring attempt. During the ladle changing operation, the tundish is kept at a full liquid level throughout the pouring process to avoid drastic fluctuations in liquid level that could cause turbulent molten steel flow and uneven stress on the billet shell. Simultaneously, the central molten steel feeding channel of the billet remains unobstructed, addressing both crack prevention and porosity / shrinkage improvement.

[0031] During the casting process, the addition of protective slag to the crystallizer is standardized. Before production, the protective slag is transferred from the ton bag to the automatic slag hopper for standby. Addition is primarily done via an automatic slag feeder, supplemented manually, strictly adhering to the principles of frequent, small, and even addition. This ensures a stable slag surface in the crystallizer, prevents leakage of red-hot steel, stabilizes the temperature and lubrication of the molten steel within the crystallizer, avoids localized overheating which exacerbates solute segregation, and reduces friction between the billet shell and the inner wall of the crystallizer, lowering the risk of internal and surface cracks. Simultaneously, the tundish steel is protected and its purity is controlled. After the tundish level reaches 200mm, 20kg of tundish covering agent is added to each stopper rod in the stabilization zone. Once the tundish is full of molten steel, 20kg of covering agent is added to each side of the long nozzle. After each heat is cast, an additional 10kg is added to each side of the long nozzle. Throughout the process, the molten steel is isolated from air to prevent secondary oxidation and the formation of oxide inclusions, further reducing solute element segregation and accumulation.

[0032] During the steady-state casting stage of continuous casting, core process parameters are dynamically matched with the superheat of the molten steel in the tundish to achieve targeted defect control: the casting speed is linked to the superheat, with the casting speed controlled at 0.65 m / min when the superheat is below 15℃, 0.60 m / min when the superheat is between 15-45℃, and reduced to 0.55 m / min when the superheat is above 45℃. By matching the casting speed with low superheat, the wide mushy region of high-carbon steel is shortened, the dendritic network's obstruction of the feeding channel is broken, the central porosity shrinkage cavity is alleviated, and the solute element enrichment time is shortened, suppressing central segregation; the cooling water flow rate of the crystallizer is fixed at 2450 L / min to ensure uniform billet shell growth, and the electromagnetic stirring parameters of the crystallizer are set... The electromagnetic stirring parameters are set at 150A / 2Hz, with a final electromagnetic stirring parameter of 400A / 6Hz. The alternating stirring cycle is 10s running, 3s pausing, 10s running, and 3s pausing. Electromagnetic force is used to break up the developed dendrites and disperse the solute mother liquor enriched in the central region, promoting uniform component distribution and optimizing the central feeding conditions. A static reduction process is also provided, with reduction parameters set at 3 / 5 / 5 / 5 / 5mm and a total reduction of 23mm. Precise pressure is applied at the end of the billet solidification to squeeze out residual pores in the center, forcibly feeding the unsolidified molten steel, completely eliminating the loose shrinkage cavities in the center, and at the same time offsetting the tensile stress generated by solidification shrinkage, avoiding stress exceeding the standard and forming central cracks, thus comprehensively improving the central density and overall quality stability of the billet.

[0033] Example 2

[0034] This embodiment provides a method for controlling the center quality of high carbon steel continuous casting billets. It adopts a standard process route of converter smelting-refining-vacuum, and is equipped with tundish pretreatment and a five-machine, five-strand 250mm×280mm continuous casting machine for production, with closed-loop quality control throughout the process.

[0035] The chemical composition of the high-carbon steel continuous casting billet in this embodiment, by weight percentage, includes: C 1.00%, Si 0.28%, Mn 0.90%, P 0.020%, S 0.030%, Alt 0.045%, Cr 0.35%, Ti 0.0050%, Mo 0.10%, O 0.0010%, Ni 0.20%, Cu 0.20%, As 0.04%, Sn 0.0150%, Pb 0.002%, Bi 0.0030%, Sb 0.0100%, V 0.05%, with the remainder being Fe and unavoidable impurities.

[0036] The specific steps of the high-carbon steel continuous casting billet center quality control method in this embodiment are as follows:

[0037] Material preparation stage: 82 tons of molten iron and 25 tons of scrap steel are added to each furnace. The composition of the molten iron is controlled as follows: C 4.2%, Si 0.45%, S 0.035%, Ti 0.05%, P 0.075%. The temperature of the molten iron entering the furnace is 1320℃. The steel output of the converter is 100t. After smelting, the steel enters the refining and vacuum treatment processes in sequence.

[0038] Refining control: The refining time is 65 minutes. The total time from the end of refining to the hoisting of molten steel to the continuous casting pouring position is consistent with the single-heat pouring cycle of the continuous casting machine, ensuring that the molten steel level in the tundish does not fluctuate significantly and the superheat is stable, enabling uninterrupted pouring of multiple heats. The composition of the finished steel is controlled at C 0.65%, P 0.008%, and S 0.04%, with strict control over the content of harmful elements.

[0039] Intermediate ladle treatment: sprue length 640mm, flow spacing 1300mm; gradient baking: low heat baking for 2.5h at 280℃, low heat for 1h covering 1 / 3 of the ladle wall with flame, low heat for 2h covering 2 / 3 of the ladle wall, medium heat baking until the bottom of the ladle shows red flame, then high heat for heat storage, total baking time 7h; stopper rod baking for 3.5h, immersion sprue baking for 2.5h.

[0040] Spraying pretreatment: Y-type filters are installed in the spraying pipes. Before starting the watering, wastewater is drained for testing. The spraying water quality is controlled at pH 11, turbidity 25 NTU, and calcium hardness 35 mg / L to ensure uniform spraying from the nozzles.

[0041] Continuous casting: After the nozzle is aligned and adjusted to be qualified, casting begins. Once the liquid level stabilizes and the nozzle turns red, the flow is manually controlled for 65 seconds, then switched to automatic control. The ladle is changed to maintain full liquid level throughout the process. STH-3 special mold flux for large square billets and high carbon steel is added. The main components of the mold flux include CaO 25wt%, SiO2 35wt%, Al2O3 35wt%, MgO 3wt%, and F. -7wt% and fixed carbon 15wt%, melting point 1075-1100℃. Add frequently in small amounts, ensuring the slag surface does not turn red; when the liquid level in the tundish is 200mm, add 20kg of covering agent to each stopper rod. The main components of the tundish-specific covering agent include 5wt% CaO, 50wt% SiO2, 4.5wt% Al2O3, 22wt% fixed carbon, and 13wt% MgO, with a melting point of 1350℃; add 20kg to each side of the long nozzle when the tundish is full, and add 10kg to each side after pouring begins.

[0042] Core continuous casting parameters: tundish superheat 25℃, casting speed 0.60m / min; crystallizer water flow 2450L / min, crystallizer electromagnetic stirring 150A / 2Hz, end electromagnetic stirring 400A / 6Hz, alternation cycle 10s-3s-10s-3s; static reduction 3 / 5 / 5 / 5 / 5mm, total reduction 23mm.

[0043] Example 3

[0044] This embodiment provides a method for controlling the center quality of high carbon steel continuous casting billets. It adopts the standard process route of electric furnace smelting-refining-vacuum, and is equipped with tundish pretreatment and a five-machine, five-strand 250mm×280mm continuous casting machine for production, with closed-loop quality control throughout the process.

[0045] The chemical composition of the high-carbon steel continuous casting billet in this embodiment, by weight percentage, includes: C 1.00%, Si 0.28%, Mn 0.90%, P 0.020%, S 0.030%, Alt 0.045%, Cr 0.35%, Ti 0.0050%, Mo 0.10%, O 0.0010%, Ni 0.20%, Cu 0.20%, As 0.04%, Sn 0.0150%, Pb 0.002%, Bi 0.0030%, Sb 0.0100%, V 0.05%, with the remainder being Fe and unavoidable impurities.

[0046] The specific steps of the high-carbon steel continuous casting billet center quality control method in this embodiment are as follows:

[0047] Material preparation stage: 83 tons of molten iron and 26 tons of scrap steel are added to each furnace. The composition of the molten iron is controlled as follows: C 4.1%, Si 0.50%, S 0.032%, Ti 0.045%, P 0.07%. The temperature of the molten iron entering the furnace is 1350℃. The steel output of the electric furnace is 101t. After smelting, the steel enters the refining and vacuum treatment processes in sequence.

[0048] Refining control: The refining time is 70 minutes, and the refining rhythm is synchronized with the continuous casting speed to ensure continuous casting without waiting; the composition of the finished steel is controlled at C 0.70%, P 0.007%, and S 0.038%, and the content of harmful elements is strictly controlled.

[0049] Intermediate ladle treatment: sprue length 642mm, flow spacing 1302mm; gradient baking: low heat baking for 3 hours at 270℃, low heat for 1 hour covering 1 / 3 of the ladle wall with flame, low heat for 2 hours covering 2 / 3 of the ladle wall, medium heat baking until the bottom of the ladle shows red flame, then high heat to store heat, total baking time 6.5 hours; stopper rod baking for 3 hours, immersion sprue baking for 2.5 hours.

[0050] Spraying pretreatment: Y-type filters are installed in the spraying pipes. Before starting the watering, sewage is drained and test water is tested. The spraying water quality is controlled at pH 11.5, turbidity 20 NTU, and calcium hardness 30 mg / L to ensure uniform spraying from the nozzles.

[0051] Continuous casting: After the nozzle is aligned and adjusted to be qualified, casting begins. Once the liquid level stabilizes and the nozzle turns red, the flow is manually controlled for 70 seconds, then switched to automatic control. The ladle is changed to maintain full liquid level throughout the process. STH-3 special mold flux for large square billets and high carbon steel is added. The main components of the mold flux include CaO 25wt%, SiO2 35wt%, Al2O3 35wt%, MgO 3wt%, and F. - 7wt% and fixed carbon 15wt%, melting point 1075-1100℃. Add frequently in small amounts, ensuring the slag surface does not turn red; when the liquid level in the tundish is 200mm, add 20kg of covering agent to each stopper rod. The main components of the tundish-specific covering agent include 5wt% CaO, 50wt% SiO2, 4.5wt% Al2O3, 22wt% fixed carbon, and 13wt% MgO, with a melting point of 1350℃; add 20kg to each side of the long nozzle when the tundish is full, and add 10kg to each side after pouring begins.

[0052] Core continuous casting parameters: tundish superheat 30℃, casting speed 0.65m / min; crystallizer water flow 2450L / min, crystallizer electromagnetic stirring 150A / 2Hz, end electromagnetic stirring 400A / 6Hz, alternation cycle 10s-3s-10s-3s; static reduction 3 / 5 / 5 / 5 / 5mm, total reduction 23mm.

[0053] Example 4

[0054] This embodiment provides a method for controlling the center quality of high carbon steel continuous casting billets. It adopts a standard process route of converter smelting-refining-vacuum, and is equipped with tundish pretreatment and a five-machine, five-strand 250mm×280mm continuous casting machine for production, with closed-loop quality control throughout the process.

[0055] The chemical composition of the high-carbon steel continuous casting billet in this embodiment, by weight percentage, includes: C 1.00%, Si 0.28%, Mn 0.90%, P 0.020%, S 0.030%, Alt 0.045%, Cr 0.35%, Ti 0.0050%, Mo 0.10%, O 0.0010%, Ni 0.20%, Cu 0.20%, As 0.04%, Sn 0.0150%, Pb 0.002%, Bi 0.0030%, Sb 0.0100%, V 0.05%, with the remainder being Fe and unavoidable impurities.

[0056] The specific steps of the high-carbon steel continuous casting billet center quality control method in this embodiment are as follows:

[0057] Material preparation stage: 81 tons of molten iron and 24 tons of scrap steel are added to each furnace. The composition of the molten iron is controlled as follows: C 4.3%, Si 0.40%, S 0.038%, Ti 0.055%, P 0.078%. The temperature of the molten iron entering the furnace is 1330℃. The steel output of the converter is 99t. After smelting, the steel enters the refining and vacuum treatment processes in sequence.

[0058] Refining control: The refining time is 60 minutes, and the refining rhythm is synchronized with the continuous casting speed to ensure continuous casting without waiting; the composition of the finished steel is controlled at C 0.68%, P 0.009%, and S 0.042%, and the content of harmful elements is strictly controlled.

[0059] Intermediate ladle treatment: sprue length 638mm, flow spacing 1298mm; gradient baking: low heat baking for 2.7h at 290℃, low heat for 1h covering 1 / 3 of the ladle wall with flame, low heat for 2h covering 2 / 3 of the ladle wall, medium heat baking until the bottom of the ladle shows red flame, then high heat to store heat, total baking time 7.5h; stopper rod baking for 3.8h, immersion sprue baking for 2.8h.

[0060] Spraying pretreatment: Y-type filters are installed in the spraying pipes. Before starting the watering, sewage is drained and test water is tested. The spraying water quality is controlled at pH 10.5, turbidity 28 NTU, and calcium hardness 38 mg / L to ensure uniform spraying from the nozzles.

[0061] Continuous casting: After the nozzle is aligned and adjusted to be qualified, casting begins. Once the liquid level stabilizes and the nozzle turns red, the flow is manually controlled for 60 seconds, then switched to automatic control. The ladle is changed to maintain full liquid level throughout the process. STH-3 special mold flux for large square billets and high carbon steel is added. The main components of the mold flux include CaO 25wt%, SiO2 35wt%, Al2O3 35wt%, MgO 3wt%, and F. -7wt% and fixed carbon 15wt%, melting point 1075-1100℃. Add frequently in small amounts, ensuring the slag surface does not turn red; when the liquid level in the tundish is 200mm, add 20kg of covering agent to each stopper rod. The main components of the tundish-specific covering agent include 5wt% CaO, 50wt% SiO2, 4.5wt% Al2O3, 22wt% fixed carbon, and 13wt% MgO, with a melting point of 1350℃; add 20kg to each side of the long nozzle when the tundish is full, and add 10kg to each side after pouring begins.

[0062] Core continuous casting parameters: tundish superheat 28℃, casting speed 0.55m / min; crystallizer water flow 2450L / min, crystallizer electromagnetic stirring 150A / 2Hz, end electromagnetic stirring 400A / 6Hz, alternation cycle 10s-3s-10s-3s; static reduction 3 / 5 / 5 / 5 / 5mm, total reduction 23mm.

[0063] Comparative Example 1

[0064] This comparative example uses a conventional continuous casting process for high-carbon steel, equipped with a five-machine, five-strand 250mm×280mm continuous casting machine, and uses the same furnace charge base as Example 2. The specific operation is as follows:

[0065] During the material preparation stage: 82 tons of molten iron and 25 tons of scrap steel were added to each furnace. The initial composition of the molten iron was not precisely controlled. Only the temperature of the molten iron was routinely tested. The temperature fluctuated between 1300-1350℃ when it entered the furnace. The steel output of the converter was 100 tons. After smelting, it directly entered the conventional refining process without vacuum deep treatment.

[0066] Refining control: The refining time is 45 minutes, with no control over the refining rhythm and continuous casting speed. After refining, the steel is directly hoisted and poured. The finished steel is only conventionally controlled with C content ≥0.6%, without precise control of phosphorus and sulfur, P content ≤0.03%, S content ≤0.08%, and no targeted impurity removal and homogenization treatment.

[0067] Intermediate tundish treatment: The sprue length and flow spacing are only routinely installed and debugged without precise dimensional control; the intermediate tundish is only routinely baked at high heat until the surface turns red, without a gradient baking process, and the total baking time is 3-4 hours. The baking time of the stopper rod and immersion sprue is not separately controlled.

[0068] Spraying pretreatment: The spraying pipes were not equipped with Y-type filters, and water was simply passed through before watering began. There was no precise water quality control or spray uniformity testing, which could lead to uneven spraying in some areas and nozzle blockage.

[0069] Continuous casting: Conventional start-up operation, after the liquid level stabilizes, directly switch to automatic control, without manual flow control transition. When changing tundishes, the liquid level in the tundish is allowed to fluctuate to more than 1 / 2. Ordinary general-purpose mold flux is used, with no fixed composition or basicity requirements, and can be added arbitrarily. The slag surface condition is not strictly controlled. The tundish is only covered with conventional bulk covering agent, with no precise composition or addition control, and no layered quantitative replenishment is performed.

[0070] Key continuous casting parameters: The superheat of the tundish is not precisely controlled and is maintained in a wide range of 40-50℃; the casting speed is fixed at 0.60m / min without any linkage adjustment; the electromagnetic stirring of the crystallizer and the electromagnetic stirring at the end are not turned on; the static reduction process is not set; the water volume of the crystallizer is conventionally controlled to fluctuate between 2400-2500L / min, without precise and constant control.

[0071] Figure 1 shows a cross-section of the high-carbon steel continuous casting billet prepared in Example 2; Figure 2 shows a cross-section of the high-carbon steel continuous casting billet prepared in Comparative Example 1. As can be seen from Figure 1, the surface and near-center region of the high-carbon steel continuous casting billet cross-section are dense and uniform, with only a very small number of tiny, scattered pinholes. The defects are small in size, sparsely distributed, and without obvious aggregation or interconnection. The overall appearance of the continuous casting billet is smooth, without large areas of roughness, pitting, or cracks, indicating high microstructure density and effective control of gas evolution and shrinkage defects. However, Figure 2 shows a large number of obvious depressions, pits, and loose areas on the surface and near-center region of the high-carbon steel continuous casting billet cross-section. The defects are larger in size and more densely distributed, with some areas showing contiguous defects forming a rough and uneven surface. Deep shrinkage cavities or subcutaneous bubble traces are visible in some areas, and the microstructure density is significantly lower than in Figure 1, indicating that shrinkage and gas evolution problems have not been effectively suppressed.

[0072] The above comparison shows that the control method provided by the present invention can effectively suppress the generation of defects such as pinholes and shrinkage cavities during the continuous casting process of high carbon steel, and significantly improve the internal density and surface quality of the continuous casting billet.

Claims

1. A method for center quality control of high-carbon steel continuously cast billets, characterized in that, The process route is converter / electric furnace-refining-vacuum-continuous casting; the composition of the finished steel is controlled as follows: C not less than 0.6%, P ≤ 0.010%, S ≤ 0.05%; the casting speed is controlled in conjunction with the superheat of the molten steel in the tundish: 0.65 m / min when the superheat is <15℃, 0.60 m / min when the superheat is 15~45℃, and 0.55 m / min when the superheat is >45℃; the cooling water flow rate of the crystallizer is 2450 L / min, the electromagnetic stirring parameter of the crystallizer is 150 A / 2 Hz, the electromagnetic stirring parameter of the end is 400 A / 6 Hz, and the alternating stirring cycle is 10 s running-3 s pause-10 s running-3 s pause; the static light reduction parameters are 3 / 5 / 5 / 5 / 5 mm, and the total reduction is 23 mm; after the start of continuous casting, manual flow control is performed for no less than 60 s before automatic control of the liquid level is introduced, and the tundish is kept full during the ladle changing process.

2. The method for center quality control of high-carbon steel continuous casting billets according to claim 1, characterized in that, The initial composition of the molten iron shall meet the following requirements: C not less than 4.0%, Si: 0.30~0.60%, S≤0.040%, Ti≤0.060%, P≤0.080%, and the temperature of the molten iron entering the furnace shall be ≥1300℃.

3. The method for center quality control of high-carbon steel continuously cast billets according to claim 1 or 2, characterized in that, The superheat of the molten steel is controlled within the range of 25~35℃.

4. The method for center quality control of high-carbon steel continuously cast billets according to claim 3, characterized in that, The refining time is no less than 60 minutes. The total time from the end of refining to the hoisting of molten steel to the continuous casting pouring position is consistent with the single-heat pouring cycle of the continuous casting machine, ensuring that the molten steel level in the tundish does not fluctuate significantly and the superheat is stable, so as to achieve uninterrupted continuous pouring of multiple heats.

5. The method for center quality control of high-carbon steel continuously cast billets according to claim 4, characterized in that, The length of the tundish nozzle is 640±5mm, and the nozzle spacing is 1300±5mm.

6. The method for center quality control of high-carbon steel continuously cast billets according to claim 5, characterized in that, The intermediate ladle adopts a gradient baking process: the low-heat baking time is not less than 2.5 hours, the low-heat baking temperature is ≤300℃, the flame covers 1 / 3 of the ladle wall for the first hour of low-heat baking, and the flame covers 2 / 3 of the ladle wall for 1~2.5 hours of low-heat baking; the medium-heat baking is carried out until the flame reaches the bottom of the ladle and produces back flame and the working layer at the bottom of the ladle turns red, and then the high-heat baking with air is turned on to bake to the specified heat storage temperature; the total baking time of low-heat, medium-heat and high-heat baking is ≤8 hours; the stopper rod is baked separately for ≤4 hours, and the immersion gate is baked separately for ≤3 hours.

7. The method for center quality control of high-carbon steel continuously cast billets according to claim 6, characterized in that, Y-type filters are installed in the continuous casting spray pipes, and the spray water quality is controlled to be pH 10~12, turbidity ≤30NTU, and calcium hardness ≤400mg / L.

8. The method for center quality control of high-carbon steel continuously cast billets according to claim 7, characterized in that, Before continuous casting begins, the spray pipes are drained of sewage and water, and a spray test operation is completed to ensure that the spray from each nozzle is uniform.

9. The method for center quality control of high-carbon steel continuously cast billets according to claim 8, characterized in that, A special mold flux for large square billets and high carbon steel, model STH-3, is added to the crystallizer. The main components of the mold flux include CaO 22~25wt%, SiO 230~35wt%, Al 2O 33~5wt%, MgO 1~3wt%, and F. - 5~7wt% and fixed carbon 11~15wt%, melting point 1075-1100℃, add frequently, in small amounts, and evenly, so that the slag surface does not leak red steel.

10. The method for center quality control of high-carbon steel continuously cast billets according to claim 9, characterized in that, A special covering agent for high-carbon steel continuous casting tundish is used. The main components of the tundish covering agent include CaO 3~5wt%, SiO2 44~50wt%, Al2O3 3~4.5wt%, fixed carbon 20~22wt%, and MgO 11~13wt%, with a melting point of 1350℃. After the liquid level in the tundish reaches 200mm, 20kg of the covering agent is added to each stopper rod. After the tundish is full, 20kg is added to each side of the long nozzle. After each furnace is started, 10kg is added to each side.

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